Voltage regulation circuit

ABSTRACT

A voltage regulation circuit for maintaining the RMS voltage supplied to a load substantially constant compared to the RMS line voltage. A triac connected in series with an AC load is gated to conduction for more than 180* but less than 360* of each cycle by means of a trigger circuit including an operational amplifier which sums the voltage developed across a capacitor during the first half cycle of the AC input voltage with a reference voltage to produce a varying output voltage which controls the length of time that the triac conducts during the second half cycle and during the next succeeding first half cycle of the input signal.

8 United States Patent 1 1 3,714,547

Ross, Jr. et a1. Jan. 30, 1973 54] VOLTAGE REGULATION CIRCUIT 3,573,6044/1971 Lundgreen ..323/22 sc x 3,440,517 4/1969 Page et al [75]Inventors: Theron A. Ross, Jr., Rochester; 2 7 9 1969 k 7 Jon w.Youngman, M e o b t 3,465, 3 Broo mire ..323/21 Primary Examiner-GeraldGoldberg [73] Assignee: Eastman Kodak Company, Rochy- Kline613lester,N.Y.

57 ABSTRACT [22] Filed: July 16,197] I i A voltage regulation circuitfor maintaining the RMS [21] Appl" l63l59 voltage supplied to a loadsubstantially constant compared to the RMS line voltage. A triacconnected in 52 u.s. C1. ..323/24, 307/252 T, 307/297, Series with an ACload is gated w conduction for more 323 3 than 180 but less than 360 ofeach cycle by means of 51 Int. Cl. ..oosr 1/32 a trigger Circuitincluding an Operational amplifier 53 Fidd f Search 323 9 22 SC, 24 2 3334; which sums the voltage developed across a capacitor 321 13; 307 297252 N 252 2 252 T, 252 during the first half cycle of the AC inputvoltage with B a reference voltage to produce a varying output voltagewhich controls the length of time that the triac [5 R f n Cited conductsduring the second half cycle and during the I next succeeding first halfcycle of the input signal. UNITED STATES PATENTS 5 Claims, 1 DrawingFigure 3,646,439 2/1972 Broski ..323/22 SC X 3,648,437 3/1972 Bridges..323/22 SC X 3,501,771 3/1970 Miller et a1. ..323/22 SC PATENTEUJAHBO191s 3'71 4' 547 JON W. YOUNGMAN INVENTOR$ BYAJM {77M ATTORNEYS THERONA. ROSS, JR.

VOLTAGE REGULATION CIRCUIT BACKGROUND OF THE INVENTION A 1. Field of theInvention This invention relates to voltage regulation circuits and moreparticularly to an improved circuit for maintaining the RMS voltagesupplied to an AC load substantially constant despite variations in theRMS value of the input signal from an AC source.

2. Description of the Prior Art In many applications it is desirablethat the RMS voltage supplied to an AC load be maintained substantiallyconstant despite relatively wide variations in the line voltage. Forexample, in microfilm readers it is desirable that the projection lampbe maintained at a constant voltage to avoid variations in screenillumination and attendant eye strain. It is also desirable that anyfluctuation in line voltage be compensated for during each cycle in asimple and efficient manner. Many prior art voltage regulation circuitshave either been extremely complex or utilized expensive electroniccomponents.

SUMMARY OF THE INVENTION According to the present invention a voltageregulationcircuit is provided for maintaining a substantially constantRMS voltage to an AC load which includes a triac connected in serieswith the load. The triac is caused to conduct during the entire firsthalf cycle of the AC input signal and during a portion only of thesecond half cycle of the input signal as a function of the magnitude ofthe AC line voltage during the first half cycle. According to an aspectof the invention, a trigger circuit for gating the triac includes anoperational amplifier which compares the voltage developed across acapacitor during one half cycle of the AC input voltage with a referencevoltage to control the triac conduction during the second half cycle ofthe AC input voltage and during the next succeeding one half cycle ofthe AC input voltage;

It is thus an object of the present invention to provide an AC voltageregulation circuit which maintains a substantially constant RMS voltageto an AC load.

The invention and its objects and advantages will become more apparentin the detailed description of the preferred embodiment presented below.

BRIEF DESCRIPTION OF THE DRAWING The above objects and advantages of thepresent invention will become apparent to those skilled in the art fromthe following detailed description read in con-' junction with theattached drawing wherein:

The sole FIGURE is an electrical schematic diagram The secondary winding24 of transformer 14 supplies a stepped down voltage to an AC load suchas lamp 26 connected in series with triac 28. Triac 28 is basically abi-directional current carrying device with conduction in bothdirections being controlled by gate 30. By applying an appropriatesignal to gate 30 each proportion of the cycle in which current flowscan be controlled to produce a substantially constant RMS voltagethrough lamp 26 despite wide variations in the voltage of source 10.

A trigger circuit 32 for supplying a gating signal to, triac gate 30includes an operational amplifier 34 having input resistors 36 and 38and resistor 40 connected between input terminal 42 and output terminal44 of amplifier 34. The output of amplifier 34 is connected to the base46 of transistor 48 by means of resistor 50, with the collector 52 oftransistor 48 being connected to gate 30 by means of resistor 54.

Connected in series across the output of secondary winding 24 are diode56, resistor 58 and diode 60 which develop a reference voltage atterminal 62 at the input to amplifier input resistor 36. As will bedescribed in greater detail below, a sample voltage will be developed atterminal 64 at the input to amplifier input resistor 38 by means of acircuit comprising diode 66, resistor 68 and resistor 70'c'onnected inseries across secondary 24, diodes 72 and 74, capacitor 76, resistor 78and variable resistor 80. Resistor 80 functions as a control to adjustthe illumination of lamp 26.

A power supply circuit 82 for supplying power to transistor 48 andamplifier 34 comprises diodes 84, 86

and 88, resistors 90, 92, and 94, capacitators 96 and 98 and switch 100.

A current limiting resistor 102 is provided across lamp 26 and resistor104 and capacitor 106 are provided across triac 28.

In operation, after the circuit has stabilized, during a positive halfcycle AC voltage from source 10 supplied by transformer 14 to the seriescircuit formed by lamp 26 and triac 28 will cause current to flow inlamp 26 during the entire half cycle since the output of amplifier 34 isat a level such that transistor 48 conducts for the entire half cyclethus gating triac 28 to conduction at the beginning of the half cycle.During the positive cycle current will also flow through diode 66,resistor 68 and diode 72 to charge capacitor 76 to a voltage of anembodiment of voltage regulation circuit according to the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT determined by the values ofresistors 68 and which form a voltage divider.

During the negative half cycle, triac 28 will block the flow of currentthrough lamp 26 until gate 30 has been triggered. After gate 30 has beentriggered, it has no further control over conduction of triac28 duringthe remainder of the half cycle. The time at which gate 30 is triggeredwill be dependent on the voltage to which capacitor 76 has been chargedduring the positive half cycle which in turn is dependent upon the inputvalue of the line voltage of source 10.

During the negative half cycle capacitor 76 discharges through resistors78 and to develop a sample voltage at terminal 64. This sample voltageis compared with a reference voltage at terminal 62 by multiplying eachvoltage by a constant and adding them algebraically by means ofoperational amplifier 34. The output voltage of amplifier 34 will be afunction of the following voltage summation:

X sample voltage During the positive half cycle of the AC input voltagedeveloped across the output of transformer 14, the output of amplifier34 will be more positive than a certain predetermined value V dependentupon the particular amplifier used, transistor 48 will be biassed toconduction the triac 28 will have been triggered to conduction at thebeginning of the positive half cycle. During the negative half cycle,the output of amplifier 34 will be initially more negative than V,transistor 48 will be non-conducting and triac 28 will also benon-conducting. As the negative half cycle continues, the sample voltagedecreases causing the output of amplifier 34 to become less negative.When the output of amplifier 34 becomes approximately equal to V, thebase-emitter junction of transistor 48 is forward biassed. Transistor 48starts to conduct, supplying gate current to triac gate 30 and causingtriac 28 to conduct for the remainder of the negative half cycle. Thus,the lamp current flows for the positive half cycle and some portion ofthe negative half cycle.

If the line voltage of source increases, thus increasing the voltagesupplied by transformer 14 to lamp 26, the sample voltage will alsoincrease and capacitor 76 will be charged to a higher potential.Capacitor 76 will take longer to discharge through resistors 78 and 80during'the negative half cycle to a value which causes the output ofamplifier 34 to equal V. Thus, the triac is triggered into conductionlater in the negative half cycle and current will flow for a lesserperiod of time through lamp 26. This effectively lowers the RMS voltageacross the-lamp.

If, on the other hand, the line voltage of source 10 decreases, thusdecreasing the voltage supplied by transformer 14 to lamp 26, capacitor76 will be charged to a lower potential. During the negative half cycle,capacitor 76 will take a shorter time to discharge and triac 26 will begated to conduction earlier in the cycle, allowing current to flow for agreater period of time and thus effectively increasing the RMS voltageacross lamp 26.

Thus, it is seen that fluctuation in the line voltage of source 10 willbe corrected for in the same cycle that the fluctuation takes place.Consequently, a substantially constant RMS voltage will be supplied tolamp 26 despite relatively wide fluctuations in the RMS value of theline voltage.

Switch 100 is a switch which may, for example, be activiated byinsertion of microfilm into a microfilm reader. When switch 100 isclosed, i.e. moved to the position shown in FIG. 1, power will besupplied to transistor 48 and amplifier 34. Because of the action of theelements of power circuit 82, the current through lamp 26 will increaseto a much lower multiple of its rated current than would normally betrue, thus minimizing line current surges when lamp 26 is turned on. Ascapacitor 96 charges the lamp illumination increases slowly andstabilizes after approximately five cycles.

Thus, there is seen that there is provided a new and improved voltageregulation circuit which supplies a substantially constant RMS voltageto an AC load despite relatively wide variations in the AC line voltage.In addition variations in line voltage are compensated for within thesame cycle that they occur thus increasing the efficiency of thecircuit.

The invention has been described in detail with particular reference toa preferred embodiment thereof, but it will be understood thatvariations and modifications can be effected within the spirit and scopeof the invention.

We claim:

1. A regulation circuit comprising:

input circuit means connectable to an AC signal source;

output circuit means connectable to an AC load;

bidirectional current carrying means connected between said inputcircuit means and said output circuit means, said current carrying meansbeing switchable in a first mode between a nonconductive state and aconductive state to carry current in a first conduction direction duringa first half-cycle of an AC input signal and being switchable in asecond mode between a non-conductive state and a conductive state tocarry current in a second conduction direction during a secondhalf-cycle of said input signal;

control means connected to said current carrying means for selectivelycausing said current carrying means to switch between a nonconductivestate and a conductive state in either of said first or second modes;

operational amplifier means having an output terminal connected to saidcontrol means and having an input terminal,

means connected to said input terminal of said operational amplifiermeans for providing a reference voltage;

means connected to said input circuit means and to said input terminalof said operational amplifier means for developing a sample voltageproportional to the input voltage during the first halfcycle of an ACinput signal;

said operational amplifier means summing said reference voltage and saidsample voltage during the second half-cycle of said AC input signal toproduce at said amplifier means output terminal a varying output signalwhich in said second mode causes said control circuit means to switchsaid current carrying means between the nonconductive state and theconductive state at a conduction angle which is a function of the sum ofsaid reference voltage and said sample voltage, such that said currentcarrying means carries current in said second conduction directionduring a portion only of said second half-cycle of said AC input signaland which in said first mode causes said control circuit means to switchsaid current carrying means between said nonconductive state and saidconductive state at the beginning of the next succeeding firsthalf-cycle of said AC input signal, such that said current carryingmeans carries current in said first conduction direction during theentire next succeeding first half-cycle of said AC input signal.

2. The regulation circuit of claim 1 wherein said bidirectional currentcarrying means includes a triac having a gate.

3. The regulation circuit of claim 2 wherein said control means includesa transistorconnected to said gate.

4. The regulation circuit of claim 3 wherein said means for providing areference voltage includes a diode.

5. The regulation circuit of claim 4 wherein said means for developing asample voltage includes a 5 capacitor.

* III i

1. A regulation circuit comprising: input circuit means connectable toan AC signal source; output circuit means connectable to an AC load;bidirectional current carrying means connected between said inputcircuit means and said output circuit means, said current carrying meansbeing switchable in a first mode between a nonconductive state and aconductive state to carry current in a first conduction direction duringa first half-cycle of an AC input signal and being switchable in asecond mode between a non-conductive state and a conductive state tocarry current in a second conduction direction during a secondHalf-cycle of said input signal; control means connected to said currentcarrying means for selectively causing said current carrying means toswitch between a nonconductive state and a conductive state in either ofsaid first or second modes; operational amplifier means having an outputterminal connected to said control means and having an input terminal,means connected to said input terminal of said operational amplifiermeans for providing a reference voltage; means connected to said inputcircuit means and to said input terminal of said operational amplifiermeans for developing a sample voltage proportional to the input voltageduring the first half-cycle of an AC input signal; said operationalamplifier means summing said reference voltage and said sample voltageduring the second half-cycle of said AC input signal to produce at saidamplifier means output terminal a varying output signal which in saidsecond mode causes said control circuit means to switch said currentcarrying means between the nonconductive state and the conductive stateat a conduction angle which is a function of the sum of said referencevoltage and said sample voltage, such that said current carrying meanscarries current in said second conduction direction during a portiononly of said second half-cycle of said AC input signal and which in saidfirst mode causes said control circuit means to switch said currentcarrying means between said nonconductive state and said conductivestate at the beginning of the next succeeding first half-cycle of saidAC input signal, such that said current carrying means carries currentin said first conduction direction during the entire next succeedingfirst half-cycle of said AC input signal.
 1. A regulation circuitcomprising: input circuit means connectable to an AC signal source;output circuit means connectable to an AC load; bidirectional currentcarrying means connected between said input circuit means and saidoutput circuit means, said current carrying means being switchable in afirst mode between a nonconductive state and a conductive state to carrycurrent in a first conduction direction during a first half-cycle of anAC input signal and being switchable in a second mode between anon-conductive state and a conductive state to carry current in a secondconduction direction during a second Half-cycle of said input signal;control means connected to said current carrying means for selectivelycausing said current carrying means to switch between a nonconductivestate and a conductive state in either of said first or second modes;operational amplifier means having an output terminal connected to saidcontrol means and having an input terminal, means connected to saidinput terminal of said operational amplifier means for providing areference voltage; means connected to said input circuit means and tosaid input terminal of said operational amplifier means for developing asample voltage proportional to the input voltage during the firsthalf-cycle of an AC input signal; said operational amplifier meanssumming said reference voltage and said sample voltage during the secondhalf-cycle of said AC input signal to produce at said amplifier meansoutput terminal a varying output signal which in said second mode causessaid control circuit means to switch said current carrying means betweenthe nonconductive state and the conductive state at a conduction anglewhich is a function of the sum of said reference voltage and said samplevoltage, such that said current carrying means carries current in saidsecond conduction direction during a portion only of said secondhalfcycle of said AC input signal and which in said first mode causessaid control circuit means to switch said current carrying means betweensaid nonconductive state and said conductive state at the beginning ofthe next succeeding first half-cycle of said AC input signal, such thatsaid current carrying means carries current in said first conductiondirection during the entire next succeeding first half-cycle of said ACinput signal.
 2. The regulation circuit of claim 1 wherein saidbidirectional current carrying means includes a triac having a gate. 3.The regulation circuit of claim 2 wherein said control means includes atransistor connected to said gate.
 4. The regulation circuit of claim 3wherein said means for providing a reference voltage includes a diode.